Don't know what the F is with the quick posting. Anyway, I like that kick because I have been practicing it for literally 30 years and can land it quickly and easily but it has almost no potential of knocking anyone out. It's a distraction to follow up with a low kick on the other side or a takedown. Since there is no windup, you can just snap it up there and BOOM. Okay, more like bing. The outside is better than the inside but harder to suprise someone with. Try both on a heavy bag and you will see. The knee does not like the inside version.

"We often joke -- and we really wish it were a joke -- that you will only encounter two basic problems with your 'self-defense' training.
1) That it doesn't work
2) That it does work"
-Animal MacYoung

They are ideal for faking because of the way they take a curved track. The observer's brain essentially applies all processing power to tracking and predicting the path of the kick, and other movement, especially linear, is virtually invisible in the moment. It's one of many street magic tricks that translate well to fighting.

First, I would be wary of "real" grappling moves a karate or TKD instructor shows you that are supposedly the true purpose of a movement in a kata or drill. This is a silly game some instructors like to play. They get off when they think they've found the hidden meaning of a form. It's mostly just talk that helps them justify bullshit like impractical technique in a kata.

So, to your question about crescent kicks. It's a kick. If you kick someone in the face with a crescent kick, they will most likely not find it pleasant. So, that's good.

Most people who practice technique like this are wasting their time because they can't fight. They spend way too much time on flashy technique and not enough time on the fundamentals of fighting.

Crescent kicks have their place though. Great fighters can utilize pretty much any kick they want because they're so solid in all areas that they can employ lower percentage techniques and not get murdered if the technique fails. Think Anderson Silva.

I guess my point is that any technique can be useful, but a fighter should make these type of kicks the icing on the cake of his training. They shouldn't be relied on as the main arsenal.

And to answer your main question - I don't think there's any secret intended meaning or use of the kick. It's just another kick.

Great points (and other responses have been great, so thank you all).

I don't TKD grapple. I do Judo now, and haven't been in a TKD class in probably 7 years, it was just a personal example of the "seeming kata bullshido that actually has a purpose" that others have posted about. It was a pretty basic application of a standing arm lock that was taken completely out of context... and might have required catching a punch (I don't remember).

Nice to know there are some folks who have put crescent kicks to good use in actual pressure testing. I think a crescent style kick is about the only way I am going to pull of any higher kicks (I know, no kicking in Judo, but we are doing some combat sambo/combat jujitsu training and sparring), as I have some weird impingement in my hips (I am working on that) and can't really get much hip turnover when I kick. But I think that is also partly because I am trying to TKD style kick all the time.

I'd love more concrete examples of people using crescents in sparring etc!

I have found that rare is the occasion when you can throw an inside crescent kick where a round kick wouldn't work better, or an outside crescent kick where a hook kick wouldn't work better.
Mostly, I use them for stretching and conditioning.

The fool thinks himself immortal,
If he hold back from battle;
But old age will grant him no truce,
Even if spears spare him.

I've found that front kicks are good for turning on light switches. Actually good practice for front kicks, particularly if you do it slowly.

I can't tell you how glad I am to find that I'm not the only one who does it.

As for crescent kick, it's one of the only techniques I learned as a kid, in both Hapkido and TKD, that I still use from time to time even though it hasn't been covered in any of the styles I've trained as an adult. Mostly, like the others said, for stretching, but I've managed to score a few hits with it in sparring too. Inside crescent kick, to be precise. It comes from an unexpected angle, but does little good in the way of causing damage I find.